This invention relates to a polyphonic electronic musical instrument of a type which produces a tone by converting tone waveshape data generated in digital into an analog signal and, more particularly, to an electronic musical instrument of such type in which construction of a digital-to-analog conversion circuit is simplified and an aliasing noise, which may be caused in produced tones by processing waveshape sample point data of a tone, is removed by a relatively simple construction.
In typical prior art polyphonic type electronic musical instruments as shown for instance in the U.S. Pat. No. 4,409,876, particularly its FIG. 5, digital tone signals for plural channels are generated in time division, digital tone signals of all channels in one sample point are added together in a digital accumulator and the added digital tone signals are converted into analog signals by a digital-to-analog converter (hereinafter called D/A converter) and thereafter are supplied to a sound system. Such construction, however, has the disadvantage that the accumulator must be composed of a digital circuit to add digital signals of multiple bits together resulting in a large hardware structure. Besides, since the D/A converter receives a sum of tone signals for plural channels provided by the accumulator (which necessarily becomes a multiple bit data), the D/A converter must be a large one of a multiple input type.
Furthermore, in the above described type of electronic musical instrument which generates digital tone signals for plural channels in time division, the sampling frequency corresponding to the time division timing and the tone frequency are generally non-harmonic with each other and, accordingly, an aliasing noise which is non-harmonic with the tone frequency is produced as will be apparent from the sampling theorem. Some arrangement must be made to eliminate this aliasing noise and, in the prior art electronic musical instrument, the arrangement was made by a digital circuit with a result that the circuit construction becomes large and complicated.